A projection electron microscope observes sample surfaces in two dimensions by using an electron optical system to illuminate the sample surface with an electron beam, and then using this electron optical system to focus the secondary electrons or reflected electrons generated as a result on the detection surface of a detector. This makes it possible to reduce the scanning frequency (unlike an SEM); accordingly, the sample observation time can be shortened, and such microscopes have attracted attention as inspection devices for micro-devices such as semiconductors.
FIG. 5 shows one example of a microscope that is conceivable as such a projection electron microscope. The illuminating beam 24 emitted from the cathode 21 passes through a Wehnelt electrode, first anode 35, second anode 36, and illumination dedicated electron optical system 22, and is incident on an electromagnetic prism 23. The optical path of the illuminating beam 24 is altered by the electromagnetic prism 23; the illuminating beam 24 then passes through a cathode lens 27, and illuminates the surface of the sample 26.
When the illuminating beam 24 is incident on the sample 26, secondary electrons, back-scattered electrons and reflected electrons (referred to collectively as generated electrons 28) having a distribution corresponding to the surface shape, material distribution, variations in potential, and the like of the sample 26 are generated from the sample 26. These generated electrons 28 pass through the cathode lens 27, electromagnetic prism 23, and image focusing dedicated electron optical system 29, and are projected onto an MCP (micro channel plate) detector 30. Then, an image is projected onto a CCD camera 33 via a light-mapping optical system 32. 25 indicates a sample stage. Furthermore, the image focusing electron optical system is constructed from the image focusing dedicated electron optical system 29, electromagnetic prism 23, and cathode lens 27, and the illumination electron optical system is constructed from the illumination dedicated electron optical system 22, cathode lens 27, and electromagnetic prism 23.
In such a projection electron microscope, as is seen from FIG. 5, the optical path of the electron beam that is incident on the sample 26 and the optical path of the electron beam that is emitted from the sample 26 are switched by the electromagnetic prism 23 (E×B). Accordingly, in the optical path between the sample 26 and the electromagnetic prism 23, a Coulomb effect is generated between the electrons in the illuminating electron beam and the electrons that are generated from the sample and used for observation. Consequently, the problem of blurring of the image that is focused has been encountered.
Furthermore, the electromagnetic prism 23 generates a large astigmatic aberration. It is very difficult to adjust both the illumination electron optical system and image focusing electron optical system in order to correct this aberration. Conventionally, therefore, design and adjustment have been performed by aligning the viewpoint with the image focusing electron optical system, and the adjustment of the illumination electron optical system has often been insufficient.
Moreover, as is seen from FIG. 5, the illumination electron optical system and the image focusing electron optical system are designed as completely separate systems in a conventional apparatus; accordingly, a correspondingly larger number of electron beam optical members must be used.